Production method of ceramic honeycomb filter

ABSTRACT

A method for producing a ceramic honeycomb filter having predetermined plugs comprising immersing the end portions of a ceramic honeycomb structure having large numbers of flow paths, with its end surface covered with a mask provided with holes only at positions corresponding to the predetermined flow paths, in a plugging material slurry, so that the plugging material slurry is introduced into the end portions of the predetermined flow paths, and removing plugging material protrusions formed in the flow paths that should not be plugged.

FIELD OF THE INVENTION

The present invention relates to a method for producing a ceramichoneycomb filter suitable for removing particulate matter from anexhaust gas, etc. discharged from diesel engines.

BACKGROUND OF THE INVENTION

FIGS. 1( a) and 1(b) schematically show one example of ceramic honeycombfilters for capturing particulate matter in exhaust gases fromautomobiles. The ceramic honeycomb filter 11 comprises a ceramichoneycomb structure 1 having large numbers of flow paths 3, 4partitioned by porous cell walls 2 extending from an inlet end surface12 to an outlet end surface 13, and plugs 5, 5 for sealing flow paths 3,4 alternately in a checkerboard pattern on both end surfaces 12, 13. Theporous cell walls 2 have as high porosity as, for instance, 55-80% toavoid pressure loss increase, because they carry catalytic materials forburning particulate matter in the exhaust gas at low temperatures.

As a method for producing a ceramic honeycomb filter having suchstructure, JP2001-300922A and JP2002-28915A disclose, as shown in FIG.2, a method comprising the steps of (a) attaching a film 6 to an endsurface 12 of a ceramic honeycomb structure 1, (b) providing the film 6with holes 7 at positions corresponding to flow paths 3, (c) placing theceramic honeycomb structure 1 in a plugging material slurry 8 containingceramic particles, an organic binder, water, etc. with the end surface12 downward, (d) introducing the plugging material slurry 8 into theflow paths 3 through the holes 7 of the film 6 while pressing theceramic honeycomb structure 1 downward by a pressing means 10, (e)dewatering and hardening the plugging material slurry 8 entering intothe flow paths 3, and finally (f) peeling the film 6.

However, because the porous cell walls 2 are, for instance, as thin as0.1-0.5 mm and have as high porosity as 55-80%, they easily have chippedportions 21 on the end surface 12 as shown in FIG. 3. Also, becausethere is no sufficient adhesion between the end surfaces 12 of the thinporous cell walls 2 and the film 6, gaps are likely to be generatedbetween the end surfaces 12 and the film 6 by pressing. As a result, theplugging material slurry 8 is likely to leak to adjacent flow paths 4that should not be plugged at the end surface 12, and stick to theircell walls 2 as protrusions 51 as schematically shown in FIG. 4. Theformation of the plugging material protrusions 51 occurs on both sidesof the ceramic honeycomb structure 1. With the plugging materialprotrusions 51 formed, the flow paths 3, 4 are narrowed, resulting inpressure loss increase in the ceramic honeycomb filter 11. In addition,particulate matter in the exhaust gas is accumulated on the pluggingmaterial protrusions 51, pressure loss increases in the ceramichoneycomb filter 11 in a short period of time.

JP9-29019A discloses a method for plugging a ceramic honeycombstructure, which comprises (a) covering an end surface of a ceramichoneycomb structure with a film, (b) providing the film with holes atpositions corresponding to flow paths that should not be plugged, (c)charging a temporary plugging material slurry comprising powderinsoluble in a solvent for a plugging material slurry and burnable ordecomposable by heating, into the flow paths through the holes, (d)peeling the film from the end surface after the temporary pluggingmaterial slurry is dried, (e) charging the plugging material slurry intothe end portions of flow paths to be plugged, and (f) sintering theplugging material slurry while decomposing the resultant temporaryplugs. Because the plugging material slurry is charged into the flowpaths to be plugged after the temporary plugging material slurry ischarged into the flow paths that should not be plugged, the pluggingmaterial slurry does not leak to the flow paths that should not beplugged. However, with the temporary plugging material slurry leaking tothe flow paths to be plugged, the flow paths to be plugged areinsufficiently filled with the plugging material slurry, and the plugsare bonded to the cell walls with insufficient adhesion strength. Inaddition, this method is not efficient because it needs twoslurry-charging steps.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodfor producing a ceramic honeycomb filter free from plugging materialprotrusions in flow paths that should not be plugged, thereby sufferingsmaller pressure loss.

DISCLOSURE OF THE INVENTION

The method of the present invention for producing a ceramic honeycombfilter having predetermined plugs comprises immersing the end portionsof a ceramic honeycomb structure having large numbers of flow paths,with its end surface covered with a mask provided with holes only atpositions corresponding to the predetermined flow paths, in a pluggingmaterial slurry, so that the plugging material slurry is introduced intothe end portions of the predetermined flow paths, and removing pluggingmaterial protrusions formed in the flow paths that should not beplugged.

The plugging material protrusions are removed preferably before burningthe plugs. The removal of the plugging material protrusions ispreferably conducted by (1) removing the end portions of the ceramichoneycomb filter, (2) blowing a high-pressure gas to an end surface ofthe ceramic honeycomb filter, or (3) moving a protrusions-removing meansin a flow path in which the plugging material protrusions are formed.The protrusions-removing means is preferably an elastic member.

The protrusions-removing means is preferably a brush comprising twistedwires, and large numbers of elastic filaments held by the twisted wires.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is an end view schematically showing an example of ceramichoneycomb filters.

FIG. 1( b) is a schematic view showing the cross section of the ceramichoneycomb filter of FIG. 1( a).

FIG. 2 is a view schematically showing an example of the productionmethods of ceramic honeycomb filters.

FIG. 3 is a partial perspective view schematically showing a chippedportion in a cell wall in an end portion of the ceramic honeycombfilter.

FIG. 4 is a schematic cross-sectional view showing an example of ceramichoneycomb filters produced by conventional methods.

FIG. 5( a) is a schematic cross-sectional view showing a ceramichoneycomb filter produced by the method according to one embodiment ofthe present invention, at a stage in which end portions are not cut off.

FIG. 5( b) is a schematic cross-sectional view showing the ceramichoneycomb filter of FIG. 5( a), from which end portions are cut off toremove plugging material protrusions.

FIG. 6 is a schematic cross-sectional view showing the removal ofplugging material protrusions by the method according to anotherembodiment of the present invention.

FIG. 7 is a schematic cross-sectional view showing a brush inserted intoan end portion of a flow path.

FIG. 8( a) is a view showing an example of the cross section shapes offlow paths in ceramic honeycomb filters.

FIG. 8( b) is a view showing another example of the cross section shapesof flow paths in ceramic honeycomb filters.

FIG. 8( c) is a view showing a further example of the cross sectionshapes of flow paths in ceramic honeycomb filters.

DESCRIPTION OF THE BEST MODE OF THE INVENTION

[1] Production of Ceramic Honeycomb Filter

As shown in FIGS. 2 and 5, a plastic moldable material of ceramic powderis extrusion-molded to a green body having a honeycomb structure, andsintered to provide a ceramic honeycomb structure 1 having large numbersof flow paths partitioned by porous cell walls. A resin mask 6 usuallyin the form of a film is attached to an end surface 12 of the ceramichoneycomb structure 1 [step (a)], and provided with holes 7 by laser,etc. alternately in a checkerboard pattern such that plugs 5 are formedin flow paths 3 to be plugged [step (b)]. The end portion of the ceramichoneycomb structure 1 is immersed in a plugging material slurry 8 [step(c)], and pressed from above by a pressing means 10 to introduce theplugging material slurry 8 into the flow paths 3 through the holes 7[step (d)]. After removing the mask film 6, plugs are dried [step 2(e)].Plugs are formed also on the side of the other end surface 13. The plugs5 are then sintered to provide a plugged ceramic honeycomb filter.Incidentally, the formation of the plugs 5 may be conducted eitherbefore or after burning the ceramic honeycomb structure 1.

[2] Method of Removing Plugging Material Protrusions

On either end surface side, the plugging material slurry 8 inevitablyleaks to the flow paths that should not be plugged, through gaps betweenthe end surface 12 and the mask film 6 and chipped portions in the cellwalls 2, resulting in the formation of plugging material protrusions 51.To remove the plugging material protrusions 51, it is preferable to use(1) a method of removing end portions from the ceramic honeycomb filter11, (2) a method of blowing a high-pressure gas (for instance,high-pressure air) to the end surface of the ceramic honeycomb filter11, or (3) a method of moving a protrusions-removing means in the flowpaths in which plugging material protrusions 51 are formed.

Utilizing the phenomenon that the plugging material protrusions 51 areformed in the flow paths mainly near the end surface, the method (1)cuts both end portions [regions outside of planes 15, 15 shown in FIG.5( a)] off from the ceramic honeycomb filter 11 to remove the pluggingmaterial protrusions 51 [see FIG. 5( b)].

The method (2) blows a high-pressure gas to an end surface close to ordistant from the plugging material protrusions 51.

The method (3) inserts a protrusions-removing means 40 into an endportion of a flow path. As shown in FIG. 6, when theprotrusions-removing means 40 is caused to move in the flow path, theplugging material protrusions 51 coming into contact with theprotrusions-removing means 40 are surely detached from the cell walls ofthe flow path. The detached plugging material protrusions 52 can easilybe discharged from the flow paths by directing the opening ends of theflow paths downward, or by air blow, etc. The protrusions-removing means40 is not restricted, as long as it can be inserted into a flow path,and has such rigidity that the plugging material protrusions 51, withwhich it comes into contact, are detached from the cell walls. It maybe, for instance, a round rod made of metals, resins, ceramics, etc., ora rod having a substantially similar cross section to those of the flowpaths. The rod may have grooves on its outer surface. The rod may beelastic.

When the protrusions-removing means 40 is elastic, it can reach cornersof flow paths each having, for instance, a tetragonal cross section, sothat it can surely remove the plugging material protrusions 51. Inaddition, the elastic protrusions-removing means 40 does not causedamage to the cell walls 2 when it is brought into contact with them.

A preferred example of the protrusions-removing means is, as shown inFIG. 7, a brush 40 a comprising a pair of twisted wires 41, and largenumbers of elastic filaments 42 held by the twisted wires 41. When thebrush 40 a moves back and forth in a flow path 3, 4, the pluggingmaterial protrusions 51 in contact with the elastic filaments 42 aresurely detached from the cell walls 2 by the elasticity of the elasticfilaments 42. A tip end portion 43 of the twisted wires 41 may firstcome into contact with the plugging material protrusions 51, and moveinto the flow path 3, 4 while destroying them. Therefore, even thoughthe flow path 3, 4 is narrowed by the plugging material protrusions 51,the brush 40 a can easily be inserted. Further, the elastic filaments 42can easily reach corners of the flow path 3, 4 having, for instance, atetragonal cross section, so that the plugging material protrusions 51can surely be removed without damaging the cell walls 2.

From the aspect of strength, elasticity, elastic-filaments-holdingforce, etc., the twisted wires 41 are preferably made of metals,particularly stainless steel. To surely remove the plugging materialprotrusions 51 without damaging the cell walls 2, the elastic filaments42 are preferably made of plastic resins such as polyamides (nylon 6-10,nylon 6-12, etc.), polybutylene terephthalate, polytrimethyleneterephthalate, polyethylene terephthalate, etc. The elastic filaments 42may be constituted by two or more filaments with different rigidity.

The cross section shapes of the elastic filaments 42 are notparticularly restricted as long as they have enough rigidity forremoving the plugging material protrusions 5 1, and may be circular,hexagonal, pentagonal, tetragonal, triangular, etc. To make theinsertion of the brush 40 a into the flow path 3, 4 easy, the elasticfilament assembly 42 a may be tapered. Of course, the elastic filamentassembly 42 a may have a uniform, or regularly or irregularly changingouter diameter.

As shown in FIG. 7, the diameter D of the elastic filament assembly 42 ais preferably larger than the maximum diameter dmax of the flow path 3,4, so that the elastic filament assembly 42 a can reach corners of theflow path 3, 4. For instance, when the flow path 3, 4 has a tetragonal,triangular or hexagonal cross section, the maximum diameter dmax of theflow path 3, 4 is as shown in FIGS. 8( a), 8(b) and 8(c).

The removal of the plugging material protrusions 51 may be conductedeither before or after sintering the plugs 5, but it is preferablyconducted before burning the plugs 5, because the sintering of the plugs5 results in the sintered plugging material protrusions 51 having higheradhesion to the cell walls 2. Also, to facilitate access to the pluggingmaterial protrusions 51 from the flow path end, the removal of theplugging material protrusions 51 is preferably conducted after peelingthe mask film 6.

Though one protrusions-removing means 40 is inserted into the flow paths3, 4 in the above example, pluralities of protrusions-removing means 40may be inserted into pluralities of flow paths 3, 4 on a one-to-onebasis simultaneously.

The present invention will be explained in more detail with reference toExamples below without intention of restricting the scope of the presentinvention.

EXAMPLE 1

Powders comprising kaolin, talc, silica, aluminum hydroxide and aluminawere formulated to prepare cordierite-forming powder comprising 47-53%of SiO₂ and 32-38% of Al₂O₃ by mass. The cordierite-forming powder wasfully blended with a methylcellulose binder, a pore-forming agent, and apredetermined amount of water to form a plastic moldable material.

The moldable material was extrusion-molded, dried, and sintered at 1400°C., to produce a cordierite ceramic honeycomb structure 1 (outerdiameter: 267 mm, length: 314 mm, cell wall pitch: 1.55 mm, cell wallthickness: 0.32 mm, and cell wall porosity: 63%) having large numbers offlow paths with tetragonal cross sections.

As shown in FIG. 2, a resin mask film 6 was attached to an end surfaceof the ceramic honeycomb structure 1 [step (a)], and provided with holes7 in a checkerboard pattern by laser [step (b)]. An end portion of theceramic honeycomb structure 1 on the side of the end surface 12 wasimmersed in a plugging material slurry 8 of cordierite [step (c)], andthe plugging material slurry 8 was introduced into the end portions ofthe flow paths 3 through the holes 7 until the resultant plugs 5 becameas long as about 15 mm, while pressing the ceramic honeycomb structure 1by a pressing means 10 [step (d)]. After the mask film 6 was removed,the plugs 5 were dried [step (e)] to obtain a plugged ceramic honeycombstructure. The plugging material slurry 8 leaked from some flow paths 3to adjacent flow paths 4 that should not be plugged, through gapsgenerated by the peeling of the mask film 6 and chipped portions in thecell walls 2, resulting in the formation of plugging materialprotrusions 51 in the flow paths 4.

After plugs 5 were similarly formed on the side of the other end surface13, the plugs 5 on both sides were sintered at 1400° C. to obtain theceramic honeycomb filter 11 shown in FIG. 5( a). It was confirmed thatthere were plugging material protrusions 51 formed near each end surface12, 13. Accordingly, end portions each 5 mm from the end surface 12, 13were cut off from the filter 11 by a milling cutter to removesubstantially all plugging material protrusions 51, thereby obtainingthe ceramic honeycomb filter 11 (length: 304 mm, plug length: about 10mm) having plugs 5 only in the end portions of the flow paths to beplugged as shown in FIG. 5( b).

EXAMPLE 2

After forming a cordierite ceramic honeycomb structure 1 (outerdiameter: 267 mm, length: 304 mm, cell wall pitch: 1.55 mm, cell wallthickness: 0.32 mm, and cell wall porosity: 63%) having large numbers offlow paths 3, 4 with substantially tetragonal cross sections in the samemanner as in Example 1, a plugging material slurry 8 was charged intoboth end portions of the flow paths 3, 4 alternately in a checkerboardpattern, such that the resultant plugs 5 became as long as about 10 mm.The plugs 5 were sintered at 1400° C. to produce the cordierite ceramichoneycomb filter 11 shown in FIG. 5( a). Plugging material protrusions51 were formed in the flow paths of the filter that should not beplugged near each end surface. As a result of blowing a high-pressureair of 1 MPa to the flow paths 3, 4 on both ends of the filter 11,substantially all plugging material protrusions 51 were removed.

EXAMPLE 3

After forming a cordierite ceramic honeycomb structure 1 (outerdiameter: 267 mm, length: 304 mm, cell wall pitch: 1.55 mm, cell wallthickness: 0.32 mm, and cell wall porosity: 63%) having large numbers offlow paths 3, 4 with substantially tetragonal cross sections in the samemanner as in Example 1, plugs 5 of about 10 mm in length were formed inthe end portions of the flow paths 3 on one side. There were pluggingmaterial protrusions 51 partially formed in the flow paths 4 near theend surface 12. A drill of 1.0 mm in diameter was inserted into a flowpath 4 having a plugging material protrusion 51, and moved along theflow path while rotating to remove the plugging material protrusions 51.Plugs 5 of about 10 mm in length were then formed in the other endportions of the flow paths 4, and plugging material protrusions 51 wereremoved by the drill. The plugs 5 were finally sintered at 1400° C.

EXAMPLE 4

After forming a cordierite ceramic honeycomb structure 1 (outerdiameter: 267 mm, length: 304 mm, cell wall pitch: 1.55 mm, cell wallthickness: 0.32 mm, and cell wall porosity: 63%) having large numbers offlow paths 3, 4 with substantially tetragonal cross sections in the samemanner as in Example 3, plugs 5 of about 10 mm in length were formed inthe end portions of the flow paths 3 on one side. There were pluggingmaterial protrusions 51 partially formed in the flow paths 4 near theend surface 12. A brush 40 a comprising large numbers of elastic nylonfilaments 42 (diameter: 0.05-0.06 mm, and length: 2.5-2.7 mm) held bytwisted wires 41 as shown in FIG. 7 was inserted into a flow path 4having a plugging material protrusion 51, and moved along the flow pathwhile rotating to remove the plugging material protrusions 51. Detachedplugging material protrusions were discharged from the flow paths 4 byair blow. Plugs 5 of about 10 mm in length were also formed in the endportions of the flow paths 3 on the other side, and plugging materialprotrusions 51 were removed by the brush 40 a. The plugs 5 were finallysintered 1400° C.

EXAMPLE 5 AND 6

Plugging material protrusions 51 were removed in the same manner as inExample 4, except for changing the elastic filaments 42 of the brush 40a to those made of polybutylene terephthalate and polyethyleneterephthalate, respectively.

EXAMPLE 7

A ceramic honeycomb filter 11 was produced in the same manner as inExample 2, except that the plugging material protrusions 51 were removedbefore burning the plugs 5, and that the pressure of the high-pressureair was changed to 0.3 MPa.

COMPARATIVE EXAMPLE 1

After forming a cordierite ceramic honeycomb structure 1 (outerdiameter: 267 mm, length: 304 mm, cell wall pitch: 1.55 mm, cell wallthickness: 0.32 mm, and cell wall porosity: 63%) having large numbers offlow paths 3, 4 with substantially tetragonal cross sections in the samemanner as in Example 1, plugs 5 of about 10 mm in length were formed inthe end portions of the flow paths 3, 4 on both sides alternately in acheckerboard pattern, and sintered. Plugging material protrusions 51formed in part of the flow paths 3, 4 were not removed.

The ceramic honeycomb filters 11 of Examples 1-7 and Comparative Example1 were measured with respect to pressure loss and aparticulate-matter-capturing ratio by the following methods. Themeasurement results are shown in Table 1.

(1) Pressure Loss

Air at a flow rate of 15 Nm³/min was supplied to each ceramic honeycombfilter 11 on a pressure loss test stand to measure pressure at the inletand outlet of the filter 11, and the pressure difference between themwas used as the pressure loss. The pressure loss is expressed by arelative value, assuming that the pressure loss of the ceramic honeycombfilter 11 of Example 1 was 1.

(2) Capturing Ratio

Carbon powder having an average diameter of 0.042 μm was introduced at arate of 3 g/h into an air stream of 10 Nm³/min, which was supplied tothe ceramic honeycomb filter 11 for 2 hours. A carbon-powder-capturingratio (on a weight basis) was calculated from the amount of the carbonpowder added and the amount of the carbon powder captured by the ceramichoneycomb filter 11.

TABLE 1 Pressure Loss Capturing Ratio No. (relative value) (%) Example 11 97 Example 2 1.02 97 Example 3 1.05 97 Example 4 0.98 98 Example 50.99 97 Example 6 0.98 97 Example 7 0.92 95 Comparative 1.21 97 Example1

The ceramic honeycomb filters 11 of Examples 1 and 2, in which pluggingmaterial protrusions 51 were removed from the end portions of the flowpaths after sintering the plugs 5, suffered smaller pressure loss thanthat of Comparative Example 1 having plugging material protrusions 51remaining in the flow paths that should not be plugged, with as highcapturing ratios as 97%. The ceramic honeycomb filters 11 of Examples3-7, from which plugging material protrusions were removed beforeburning the plugs 5, suffered extremely smaller pressure loss than theceramic honeycomb filter 11 of Comparative Example 1, with a capturingratio of 95% or more, a level causing practically no problems.Particularly the ceramic honeycomb filters 11 of Examples 4-6, in whichplugging material protrusions 51 were surely removed even from thecorners of flow paths by a brush, suffered smaller pressure loss thanthat of Example 3, in which plugging material protrusions 51 wereremoved by a drill. Example 7 exhibited the lowest pressure loss becauseplugging material protrusions 51 were removed by a high-pressure airbefore burning the plugs 5, but some of the plugs 5 were removed,resulting in the lowest capturing ratio.

EFFECT OF THE INVENTION

Because the method of the present invention removes plugging materialprotrusions formed in the flow paths that should not be plugged at thetime of forming plugs, not only pressure loss increase, but alsopremature pressure loss increase due to the accumulation of particulatematter in the plugging material protrusions can be prevented. It is thuspossible to produce a ceramic honeycomb filter free from pluggingmaterial protrusions, even with cell walls having as high porosity as55-80%.

1. A method for producing a ceramic honeycomb filter havingpredetermined plugs comprising immersing the end portions of a ceramichoneycomb structure having large numbers of flow paths, with its endsurface covered with a mask provided with holes only at positionscorresponding to the predetermined flow paths, in a plugging materialslurry, so that said plugging material slurry is introduced into the endportions of the predetermined flow paths, and removing plugging materialprotrusions formed in the flow paths that should not be plugged.
 2. Themethod for producing a ceramic honeycomb filter according to claim 1,wherein said plugging material protrusions are removed before burningsaid plugs.
 3. The method for producing a ceramic honeycomb filteraccording to claim 1, wherein the end portions of said ceramic honeycombfilter are cut off to remove said plugging material protrusions.
 4. Themethod for producing a ceramic honeycomb filter according to claim 1,wherein a high-pressure gas is blown to an end surface of said ceramichoneycomb filter to remove said plugging material protrusions.
 5. Themethod for producing a ceramic honeycomb filter according to claim 1,wherein a protrusions-removing means is moved in a flow path in whichsaid plugging material protrusions are formed, to remove said pluggingmaterial protrusions.
 6. The method for producing a ceramic honeycombfilter according to claim 5, wherein said protrusions-removing means isan elastic member.
 7. The method for producing a ceramic honeycombfilter according to claim 6, wherein said protrusions-removing means isa brush comprising twisted wires, and large numbers of elastic filamentsheld by the twisted wires.